In the prior art it has often been necessary to provide a visual display using signals obtained from a plurality of sources and applied to a display device over a like plurality of circuit paths. In some applications two or more of these signals are applied to the display device at the same time, whereas in other applications two or more signals are applied to the display device at different times, but in either instance it is important that each of the resulting displays be in exact positional relationship to each other. One such application in which different displays must be in juxtaposition or superposition is in a radar display system.
A radar operator often observes multiple targets on the face of radially scanned cathode ray tube (CRT) called a plan position or display indicator. With multiple targets it is often difficult to remember the identity of each target displayed on the CRT, so it has proven desirable to display symbolic information on the CRT adjacent to the radar targets to identify the targets. The symbolic information is usually in alphanumeric and symbol form and provides the radar operator with the identity and sometimes other pertinent operational information concerning each target. Presentation of the symbolic information is accomplished by auxiliary means which receives target identification information from the radar operator or a computer and generates video signals for the display of the information.
In the radar display art it is well known to use a raster scanned CRT for display indication rather than a radial scanned CRT display. Raster scanned displays have provided a bright display upon which it is relatively easy to observe targets and symbolic information. In one radar system utilizing a raster scanned CRT the radar target video display is applied to the display indicator via a first path including the radar equipment and a scan converter which converts the conventional radial scan radar display for raster scan display. The symbolic information is computer generated and is applied to the radar display indicator via a second path.
There are several sources of error in each of the two signal paths to the radar display indicator screen which can cause the symbolic information not to be displayed on the radar screen adjacent to the associated radar targets. In the event that multiple targets are displayed near each other on the radar screen, misregistration of symbolic information and targets on the screen can result in misidentity of targets with possible dangerous consequences. Some sources of error causing misregistration include nonlinearities in amplifiers and the scan converter deflection systems as well as many other inherent faults well known in the art. It has been possible in the prior art to minimize the registration error by providing complex equipment and by requiring difficult alignment procedures, but both these approaches have been found to be prohibitively costly and time consuming.
Accordingly, there is a need in the art for apparatus to simply and accurately align a display system having multiple signals input thereto for multiple displays that must be in exact positional relationship to each other.